The 75-year-old, who has been crucial to the fight against the illegal trade in ivory and rhino horn, was found in his home with a stab wound to the neck on Sunday.

The former UN special envoy for rhino conservation had risked his life to secretly photograph and document illegal sales of ivory and rhino horns.

The US citizen had recently returned from a research trip to Myanmar. He, his wife Chryssee Martin and colleagues were in the midst of discovering ivory and rhino markets, the traffickers and the modern-day uses.

According to the BBC's Alastair Leithead from Nairobi, Bradley Martin was in the process of writing up his findings when he died.

His wife found him in their house in Langata.

Police are investigating the circumstances but suspect it was a botched robbery.Nairobi DCI chief Ireri Kamwende told local paper The Star that they are yet to identify the attackers.

He went to Kenya from the US in the 1970s when there was a surge in the number of elephants being killed for their ivory.

His work on illegal wildlife markets helped pressure China to ban the rhino horn trade in the 1990s, and domestic sales of ivory, which came into force this year.

In a major report last year from Laos, he and his colleague Lucy Vigne established that the country had the world's fastest growing ivory trade.

They risked their own safety staying at a Chinese casino inhabited by gangsters and traffickers in order to visit the illegal markets and find out the latest prices by posing as dealers.

His life's work was combating the illegal trade of wildlife and he produced a huge body of highly respected research and investigative reports.

Elephant expert and CEO of Wildlife Direct Dr. Paula Kahumbu tweeted: "It is with deep shock & horror that we learn this morning of the death of long time conservationist, Esmond Bradley Martin, whom police say died in suspicious circumstances st his home in Karen, Nairobi. Esmond led investigations into ivory & rhino horn trafficking.

"Esmond was at the forefront of exposing the scale of ivory markets in USA, Congo, Nigeria, Angola, China, Hong Kong, Vietnam, Laos and recently Myanmar. He always collaborated with Save the Elephants and worked with many of us generously sharing his findings & views.”